1. Field of the Invention
The present invention relates to a digital amplifier and a television receiving apparatus using the digital amplifier.
2. Description of the Related Art
A digital amplifier is generally configured to convert amplitude information of an analog audio waveform into time information which switches on/off of a switching unit in output. The higher this switching frequency, the larger the amount of information per unit time becomes. In addition, if the switching frequency is high, an image noise has only a small influence on an audible bandwidth. Accordingly, an audio play can be performed with fidelity up to a high frequency.
However, a consumption current appears by a through current and a charge/discharge current of a parasitic capacity for every on/off of the switching unit. Therefore, the higher a switching frequency, the larger electric power consumption becomes.
As a method for reducing electric power consumption, Japanese Patent Application Laid-Open No. 2004-72707 discusses that the frequency of a drive pulse supplied to a switching element can be reduced when no input signal is detected, or corresponding to an average level of the input signal.
Further, Japanese Patent Application Laid-Open No. 2004-363699 discusses that a delay time in a timing adjustment circuit which suppresses a through current can be controlled to change the delay time between a high tone quality mode and a low electric power consumption mode.
However, the method discussed in Japanese Patent Application Laid-Open No. 2004-72707 has a problem in that the input signal can be either a music signal that is easily affected by an image noise, or a speech signal that is hardly affected by the image noise. Accordingly, regardless of the music signal or the speech signal, when a signal level has a level not less than a fixed level, audio play is performed in a state of a high switching frequency, resulting in high electric power consumption.
Furthermore, in Japanese Patent Application Laid-Open No. 2004-363699, the delay time in the timing adjustment circuit is controlled, which suppresses the through current dependent on required signal quality and electric power consumption. However, this delay time in the timing adjustment circuit absorbs variation of a response time of the switching element. Accordingly, if the delay time is reduced to perform audio play with high fidelity, electric power consumption varies greatly depending on the response time of the switching element.